Chemical analysis systems often include fluid channels that accommodate high pressures. For example, a liquid chromatography system, such as a system designed for ultra performance liquid chromatography (UPLC®), can operate at pressure that may exceed 18,000 psi. The fluid channels in such systems may include tubing that is coupled to other components or tubing using a conventional coupling such as a standard compression fitting.
The improved performance of UPLC systems includes substantial increases in separation power. Adverse chromatographic effects such as carryover and peak tailing can result from the use of conventional couplings used to achieve fluid-tight seals and are more readily observable in system measurements. In typical couplings, the seal is formed along the side of the capillary. For example, many couplings use an annular sealing element such as a ferrule that has a conical outer surface. To form a fluid-tight coupling, a capillary having the annular sealing element displaced away from the endface is inserted into a receptacle of a coupling body. The receptacle is defined by a cylindrical bore that transitions to a conical bore and then to a smaller diameter cylindrical bore. A fluid channel extends from the surface at the bottom of the smaller diameter cylindrical bore into the coupling body. The cone angle of the conical bore is greater than the cone angle of the annular sealing element resulting in a seal along the circumferential contact between the annular sealing element and the conical surface of the conical bore. Additional force applied by a compression screw after achieving initial contact between the annular sealing element and conical bore surface results in a contact seal between the annular sealing element and the outer surface of the capillary. If the endface of the capillary is not in contact with the bottom of the cylindrical bore, the region between the outer surface of the capillary and the side wall of the smaller cylindrical bore below the circumferential contact seal represents an unswept volume. During a chromatographic measurement, analytes can become trapped in the unswept volume and gradually diffuse into the fluid flow, thereby degrading the chromatographic measurement data. Moreover, corrosion may occur at the capillary interface, leading to further degradation of chromatographic measurements.